To meet customer demand, the commercial printing industry requires the capability of producing spot colors and color images accurately and consistently. In a typical four color printer, there are numerous combinations of CMYK, especially for colors near the neutral axis that can produce the same color. This is called degeneracy in printers, offered by the addition of K separation. The number of combinations available to produce the same color goes down dramatically as we approach the boundary. Particularly, the appearance of spot colors might result in a non-smooth rendering because of the differences in halftone structures when just min-GCR or max-GCR solutions are used. When rendering a given color (Lab) on a CMYK printer, such as for spot color emulation, there is a range of CMYK values that will produce the desired Lab value. The available CMYK range is large for some colors (e.g., midtone neutrals) and small or zero for others (e.g., saturated colors). The available range is significantly greater for N-color systems (N>4). Although each CMYK value in the range will produce the desired Lab value, the printed spot colors with that recipe can differ widely in other attributes, such as graininess, mottle, color stability, ink cost, etc. Currently, the CMYK values are commonly chosen by applying a GCR strategy. Typically, this profile approximates the max-GCR case, i.e., a CMYK value is selected at or near the one containing the most K. This is often done because ink cost is a minimum for the max-GCR case, and because there is a common belief that this provides the best color stability. On the other hand, max-GCR can, for many colors, provide very poor uniformity (e.g., high mottle and graininess). Consequently, it is desirable to pick the CMYK recipe from among available recipes to optimize one or more of these attributes (graininess, mottle, color stability, ink cost, etc.). Methods for performing this optimization are computationally intensive since such techniques involve computing all possible CMYK recipes for a given color and then selecting among these.
Thus, depending on the choice of CMYK recipe, appearance of spot colors may be noisy due, for example, to the differences in uniformity and in contrast between the various separations. In spot color tests on several printers, proper selection of the C, M, Y and K separations has been shown to suppress the noise apparent in the prints, making them appear smooth. An optimized CMYK recipe is considered useful when it not only produces accurate color but also renders colors that appear smoother (less noisy). The term ‘smoothness’ broadly refers to various other smoothness related image quality parameters. The acronym NMF (Noise Mottle Frequency) refers to an image quality metric, which is a measure of ‘smoothness’.
Accordingly, what is needed in this art are computationally efficient methods for determining spot color recipes for use in spot color emulation for N-color printing, which produce spot colors which are not only accurate in color but are also smooth.